Signal whitening is a known type of signal processing task in many applications, and is a process whereby correlated or “colored” frequency components of a signal of interest are made to appear more like random (white) noise. While this invention is focused on wideband CDMA (WCDMA) systems, signal whitening can be used in other types of systems as well.
It is known that a whitening operation can improve the performance of a receiver. For example, a maximum-likelihood (ML)-based channel equalizer normally assumes that additive noise-plus-interference is white, i.e. its spectrum is flat and samples of the noise-plus-interference do not correlate with each other. If this is not the case (i.e., the noise if “colored”), some performance degradation can be expected. Whitening can also be implicitly performed by a linear minimum mean-square error (LMMSE) equalizer and, hence, the whitening operation can be applied to realize a linear equalizer. Linear equalizers are known to offer performance improvement in many radio communication applications.
The use of multiple antennas and/or oversampling in the receiver are also known to offer a performance improvement, as compared to using only a single receive antenna without oversampling.
WO 02/075950, “Interference rejection in a receiver” and WO 01/39448, “Methods, receiver devices and systems for whitening a signal disturbance in a communication signal”, each describe the use of a whitening filter in a CDMA system. WO 02/075950 and WO 01/39448 each describe a receiver where a whitening operation precedes other signal processing tasks, such as matched filtering and data symbol detection.
WO 01/39448 does not disclose a specific method to process an over-sampled or other signal in the case where multiple receive antennas are present It should also be noted that in WO 01/39448 it is desired to whiten the additive noise-plus-interference signal, and not the total signal (including both the desired and interfering signals).
On the other hand, WO 02/075950 seeks to whiten the total signal, but concentrates on spatial domain processing and not the time domain. WO 02/075950 also uses block processing of the input signal, i.e., a block or a vector of signal samples is processed simultaneously using multiplication by a whitening matrix. For example, given a signal of N samples, the whitening matrix is an N-by-N matrix that is obtained by Cholesky factorizing the signal covariance matrix. However, should one wish to address time domain signal whitening, in addition to spatial domain signal whitening, the whitening matrix may prove to be impractically large for most real-world applications.
Also representative of the prior art is a publication by Vidal et al. “Space-time front-ends for RAKE receivers in the FDD mode of UTRA”, Signal Theory and Communications Department, Universitat Politécnica de Catalunya, Barcelona, Spain (2000). FIG. 3 of this publication shows a temporal whitening block followed by a spatial whitening matrix feeding a plurality of RAKE receivers. The approach of Vidal et al., however, is to whiten the additional noise-plus-interference, and not the total received signal, and leads to the use of a maximum likelihood detector. Vidal et al. apply block processing of the signal (i.e., a matrix multiplication based operation) having the same disadvantages as explained above. Vidal et al. simplify the block whitening operation by separating temporal and spatial (noise) whitening, which is optimal only with certain assumptions as they describe. Vidal et al. state that the separated temporal (noise) whitening can be implemented by a FIR filter if a pth order Markov model is applied for temporal noise correlation, and they also point out the sub-optimality and limitations of this approach.
One of the main advantages of CDMA systems is the capability of using signals that arrive in the receiver with different time delays (multipath signals). Due to its wide bandwidth, and the use of multi-finger RAKE receiver, a CDMA receiver uses the multipath signals and combines them to provide a stronger. The RAKE receiver is essentially a set of receivers. For example, one of the receivers (each is typically referred to as a finger) searches for different multipaths and feeds the information to the other fingers. Each finger then demodulates the received signal corresponding to a strong multipath. The results of each finger are then combined together to yield a stronger signal.
The RAKE may be considered to be a filter that is matched to the operations of dispreading the received spread signal, pulse shape filtering and channel filtering. Such a matched filter maximizes the Signal-to-Interference-plus-Noise Ratio (SINR) at its output, if the interference plus noise is white.
The conventional RAKE does not operate to suppress the intra-cell and inter-cell interference. However, channel equalizers that are currently under study take into account both sources of interference.
Other problems associated with conventional CDMA RAKE receivers include a general inability to detect higher performance, higher order modulation schemes (e.g., 16-QAM) in multipath channels with high interference levels, and a requirement, during soft handover, that each base station signal to be detected have its own channel equalizer. Problems also arise during the equalization of space-time coded signals (e.g., during the reception of STTD transmissions). A problem also arises in multi-antenna systems, in that each transmit antenna requires its own equalizer at the receiver.